Electronic ballasts must be protected against overvoltage from the supply network, for example surge pulses. This usually takes place through various components that absorb the excess energy and thus limit the voltage in the ballast. However, these limits do not operate with such steep characteristics that the protection would be perfect. Consequently, such components are often overdimensioned.
In order to avoid the overdimensioning of components, it is known from DE 103 49 036 A1 to turn off power semiconductors, in order in this way to avoid high currents and voltages in and across such components. According to DE 103 49 036 A1, the time derivative of the input voltage is monitored and the transformer switch is turned off when the detected time derivative of the input voltage exceeds a prescribable threshold value. The transformer switch is thereby reliably protected against overvoltage. This has the advantage that the transformer switch need not be so highly dimensioned in terms of its voltage endurance as without this turning off.
The following statements on the prior art relate to FIG. 2 of the abovementioned DE 103 49 036 A1. However, in order to simplify comprehension, the same reference symbols have been used for the electronic ballast illustrated schematically in FIG. 1 of the various embodiments to the extent that the circuit structure corresponds to that from DE 103 49 036 A1.
A disadvantage of this known mode of procedure consists, however, in that the high voltage across the capacitor of the network filter, which is connected upstream of the transformer, that is to say across the capacitor C10, is stored for a certain time, since power is no longer drawn owing to the turning off of the transformer switch S20. The turning off of the transformer switch S20 consequently leads to the fact that the capacitor C30 is no longer recharged. The load circuit supplied from the capacitor C30 is operated further until it is turned off owing to undervoltage. It is now necessary to wait until the capacitor C10 is discharged via parasitic resistances down to an uncritical value before the transformer switch S20 can be turned on again. This limit value can be 400 V, for example. If it is detected that this 400 V threshold has been undershot, the ballast is restarted entirely. Owing to this mode of procedure, a period of approximately 1 s elapses from the turning off of the load circuit until the capacitor C10 is sufficiently discharged. The restarting of the electronic ballast lasts a further 1.2 s, and so the user has no light over a period of approximately 2.2 s—in the present example.